Metastable metal ion production in sputtering dc glow discharge plasmas: Characterization by electronic state chromatography

The sputtering de glow discharge is a stable and intense source of a wide variety of metal ions for use in ion/molecule studies. We have examined factors influencing the formation of metastable excited states of sputtered species in this device using the electronic state chromatography (ESC) technique. These determinations are essential if the glow discharge is to be used as a source of metal ions for subsequent reaction, since the electronic configuration of the metal can dramatically affect its reactivity in the gas phase. Further, the ability to manipulate excited-state production provides the means for the study of state-specific behavior of these species. We have obtained evidence in a series of experiments that indicates that the glow discharge is capable of producing several metal ions in metastable excited states as well as the ground state. Identification of specific configurations for several first-, second-, and third-row metal ions has been made on the basis of reduced zero-field mobilities in He. Results suggest that significant population of excited states higher than approximately 3.4 eV above the ion ground state does not occur in an Ar discharge for the metals examined here. Examination of the dependence of configuration distributions for several metal ions has revealed that excited-state production is sensitive to certain discharge parameters including pressure, distance to sampling orifice, and working gas composition. These results suggest that production of excited metal ions in the discharge involves energetic electrons. Further, some degree of deactivation of excited states occurs prior to extraction for ions whose excited states are collisionally coupled to lower states via interaction with the discharge gas. Metastable deactivation is enhanced in the third-row ions as a result of large spin-orbit effects.